1. Field of the Invention
Aspects of the present invention relate to a thin film transistor, a method of fabricating the same and an organic light emitting diode (OLED) display device having the same.
2. Description of the Related Art
Generally, polycrystalline silicon layers are widely used as semiconductor layers for a thin film transistor because they have fast field effect mobility, and can be applied to a high-speed operation circuit and a complementary metal-oxide-semiconductor (CMOS) circuit. Thin film transistors using the polycrystalline silicon layer are used as a switching device of an active matrix liquid crystal display (AMLCD) device, and switching and driving devices of an active matrix organic light emitting diode (AMOLED) display device.
The polycrystalline silicon (poly-Si) thin film transistors used in active matrix flat panel display devices, such as the AMLCD device or the AMOLED display device, are generally floating body poly-Si thin film transistors (TFT) in which a semiconductor layer is floated in the shape of an island. As the floating body poly-Si TFT gets smaller, a saturation region of a drain current is reduced, and the drain current is also reduced.
In general, the semiconductor layer is formed of a polycrystalline silicon layer over the entire surface of the substrate, and a photoresist pattern is formed on the polycrystalline silicon layer. After that, the polycrystalline silicon layer is etched using the photoresist pattern as a mask, and the edge region of the semiconductor layer is damaged due to an etch solution or plasma used in the etching process. Further, the characteristics of the semiconductor layer become non-uniform or are degraded due to the remaining photoresist in the edge region. For this reason, the TFT having the semiconductor layer changes in characteristics, such as a threshold voltage or s-factor, and has a hump in an I-V curve, which shows the characteristics of the TFT. Such problems are caused by the damaged edge region used as the channel region.
To solve these problems, a gate-body contact TFT in which a semiconductor layer is connected with a gate electrode has been suggested. In the gate-body contact TFT structure, a subthreshold slope value at a low gate voltage is improved, and a high drain current can be obtained at the low gate voltage. Thus, an on/off characteristic can be ensured at the low gate voltage, and a flat panel display device can be driven with low power consumption.
Conventionally, to realize a gate-body contact thin film transistor, a separately extended body contact region for making a contact with a gate electrode is formed in a conventional semiconductor layer. However, the separately extended body contact region increases the area of the semiconductor layer, which is undesirable.